Sylvain Traverso

Decision-Directed Channel Estimation and High I/Q Imbalance Compensation in OFDM Receivers

Direct-conversion architectures suffer from the mismatch between the in-phase (I) and the quadrature-phase (Q) branches, commonly called I/Q imbalance. Even low I/Q imbalances imply poor performance of orthogonal frequency division multiplexing (OFDM) systems. In this paper, we propose a new algorithm that uses both training and data symbols in a decision-directed fashion to jointly estimate the channel and compensate for high receiver I/Q imbalance. Simulation results show that our method can compensate for high I/Q imbalance values and also estimate a frequency selective channel.

A 14-Band Low-Complexity and High-Performance Synthesizer Architecture for MB-OFDM Communication

This work presents a 14-band low-complexity and high-performance synthesizer architecture for multiband orthogonal frequency-division multiplexing operating in the range of 3.1 to 10.6 GHz. The synthesizer uses a single phase-locked loop with trivial divide-by-2 circuits, single side band mixers, low-complexity filters, and multiplexers. Specifications of the synthesizer components are chosen in order to minimize the total spurs power and respect the IEEE802.15.3a recommendations.

Linearity improvement thanks to the association of Active Constellation Extension and digital predistortion for OFDM

Nonlinearity problems for power amplifiers are well known and many linearization techniques have been proposed in the literature, especially digital baseband predistortion. Orthogonal Frequency Division Multiplexing (OFDM) has a very high Peak-to-Average Power Ratio (PAPR), making it very sensitive to non linearities. We have chosen the Active Constellation Extension (ACE) to reduce the crest factor, which is not sufficient. In this paper, we show that the association of ACE with digital predistortion helps to improve the Adjacent Channel Power Ratio (ACPR) and so permits a good linearity improvement. The association is first validated in simulation. In measurements, for 42.5dBm RMS output power, which represents the nominal power, the ACPR is increased from 32dBc to 49dBc. For a given ACPR of 35dBc, the RMS output power is increased from 40.7dBm to 42.7dBm.

Parameters Selection for Filter Bank Precoded Filter Bank Multicarrier Systems

Filter Bank MultiCarrier - Offset QAM (FBMC-OQAM) is now a recognized alternative to Orthogonal Frequency Division Multiplexing (OFDM) for the transmission of multicarrier signals thanks to its improved spectral occupation. However FBMC-OQAM presents a high occurrence of power peaks. In order to reduce the occurrence of large power peaks, we present two efficient precoding methods: Filter Bank (FB) precoding and FB precoding with excess band. We propose the best choice of parameters for these methods in order to optimize performances in terms of peak power reduction, excess band, latency and overhead.

Improved Linearization of a high power amplifier to reduce spectral distortions near the saturation area

To mitigate in-band and out-of-band linearity problems due to an Orthogonal Frequency Division Multiplexing (OFDM) signal used on a high power amplifier (PA), we propose to improve a linearization technique called baseband digital predistortion. The new algorithm differently processes the samples depending on their power. Predistortion improvement is studied in simulation on a class A PA and then applied on a 50W peak power GaN PA with a 64QAM OFDM signal. Simulations and measurements show an improvement of the out-of-band linearity performance with very slight in-band linearity degradations and at the same time an improvement of the PA efficiency (until 6%-points for the PA Power Added Efficiency (PAE)). For a 40 dBc ACPR, the use of our improved predistortion increases by 1.3 dB the RMS (Root Mean Square) output power in comparison with classic predistortion.

On the Road to 5G: Comparative Study of Physical Layer in MTC Context

During the past few years, we are witnessing the emergence of 5G and its high-level performance targets. Waveform (WF) design is one of the important aspects for 5G that received considerable attention from the research community in recent years. To find an alternative to the classical orthogonal frequency division multiplexing (OFDM), several multicarrier approaches addressing different 5G technical challenges, have been proposed. In this paper, we focus on critical machine-type communications (C-MTC), which is one of the key features of the foreseen 5G system. We provide a comparative performance study of the most promising multicarrier WFs. We consider several C-MTC key performance indicators: out-of-band radiations, spectral efficiency, end-to-end physical layer latency, robustness to time and frequency synchronization errors, power fluctuation, and transceiver complexity. The investigated multicarrier WFs are classified into three groups based on their ability to keep the orthogonality: in the complex domain, e.g., most of the OFDM-inspired WFs, in the real domain like offset-quadrature amplitude modulation (QAM)-based techniques, and non-orthogonal WFs like generalized frequency division multiplexing and filter bank-based multicarrier-QAM. Finally, the performances of these WFs are thoroughly discussed in order to highlight their pros and cons and permit a better understanding of their capabilities in the context of C-MTC.

A Family of Square-Root Nyquist Filter With Low Group Delay and High Stopband Attenuation

A simple method for square-root Nyquist filter design with low group delay and high and fast decaying stopband attenuation is presented. The method is based on a frequency domain design procedure and can address various roll-off factors. The computation procedure is presented and justified analytically. Numerical results show that for a given group delay, the stopband attenuation performance of this proposed technique outperforms the most relevant existing techniques.

Joint Linearity and Efficiency Improvement for a High Power Amplifier

This letter presents the optimized way to combine predistortion and envelope tracking to improve at the same time the linearity and efficiency performances of an emitter. Applied to a realistic GaN (Gallium Nitride) Power Amplifier (PA) with a highly dynamic Orthogonal Frequency Division Multiplexing (OFDM) signal, the processing allows obtaining an improvement of 20 dB in terms of out-of-band linearity and simultaneously the efficiency is improved by a factor of 1.7 to 3 in the simulated power range. We analyze the performances of the proposed techniques association as a function of the power, showing its robustness.

A new family of filters for PAPR reduction of carrier aggregated signals

This paper proposes a new, simple and highly adaptive filter design for distortion-based peak to average power ratio (PAPR) reduction algorithms. The proposed method is based on a prototype filter design and is capable to efficiently address multiband signals. Recent practical implementations recommend the use of the equiripple (ER) filter. Numerical results show that for a given complexity and for a given PAPR reduction, the proposed method outperforms the performance of the ER filter in terms of power spectral density and error vector magnitude (EVM). Finally, the proposed method is extended to address the potential heterogeneity of the EVM specifications, even within the same band. We show that our approach provides an additional gain in terms of PAPR reduction capability.

Spectral performances of PAPR reduced FBMC/OQAM signals.

This article presents the evaluation of out-of-band spectral pollution caused by the nonlinearities of the PA at the RF front-end for FBMC/OQAM signals, as well as the improvement of those spectral performances thanks to an envelope dynamic reduction method based on precoding. We also study the in-band spectral pollution by creating notches in the spectrum, in the perspective of fragmented spectrum access. The impact on in- and out-of-band performances is compared to well-known OFDM signals, and evaluated through simulations of ACPR and NPR. Measurements on an RF platform with a commercial amplifier are presented to confirm the simulation results.